Begin by cupping the back of your left hand in the palm of your right. Place your right thumb at the bottom center of your left palm and begin to stroke upwards toward the fingers, travelling along the channels between the finger bones—that’s four strokes up the palm for each round. Use as much pressure as feels good and let your left hand and wrist be as relaxed as possible.
Last, bring your right hand again to the back of your left, but far enough over that you can wrap your right fingers around your fleshy left thumb mound. Squeeze the thumb mound and pull backwards to stretch open the left palm. If you have a partner, ask her to line her thumbs up next to each other, pointing upward, on the palm of your hand and wrap her 10 fingers around the back (“butterfly grip”). She can then gently stretch your palm by rolling her thumbs apart in a motion like opening a book.
There are a number of other ranges of motion to explore and ways to stretch your hands and fingers. I encourage you to play around until you find a stretch or technique that alleviates exactly the pain you’re experiencing. You’ll be glad you gave yourself a hand.